Feedback controlled photosensitive object detecting system

ABSTRACT

A system for detecting the presence of an object passing between a photosensitive resistance and a light source. A DC feedback amplifier connected to the photosensitive resistance controls the intensity of the light source so that the system functions properly under wide variations in ambient conditions without requiring operator adjustments.

United States Patent Mancini 1 1 Feb. 1, 1972 [S41 FEEDBACK CONTROLLED 3,517,167 6/1970 Bell ..235/61.1l PHOTOSENSITIVE OBJECT 2,649,834 8/ 1953 Sweet ..315/15l X DETECTING SYSTEM 3,167,739 1/1965 Girard et ll. .....250/205 X 3,331,012 7/1967 Aiken ..250/205 X [72] lnventor: Ronald A. Mancini, Riviera Beach, Fla. 3,354,773 11/1967 Shreve ..250/205 X 3,437,822 4/1969 Fitzsimmons.. ..250l205 X [73] Ass'gnee 3,471,700 10/1969 mm ..2so/2os x [22] Filed: Oct. 9, 1968 Primary Examiner-Robert Segal [21] Attorney-Clarence R. Patty, Jr., Walter S. Zebrowski and William .1. Simmons, Jr.

[52] U.S.Cl ..250/205,315/151 [51] .G0lj 1132,601] 1/36, 1105b 37/02 [57] v ABSTRACT [58] FreldolSearch ..315/l51;250/205 A sysem for damning the presence of an object passing between a photosensitive resistance and a light source. A DC [56] Rem-em cued feedback amplifier connected to the photosensitive resistance UNlTED STATES PATENTS controls the intensity of the light source so that the system functions properly under wide variations in ambient condiet a1. t ions without requiring operator adjustments Smura ..250/205 X Bower ..250/205 1 Claims, Drawing Figures Ill.

PATENIED FEB H972 3.639.768

SHEET 1 0F 2 II I I M UTILIZATION I35 AC AMPLIFIER DEVICE T (PRIOR ART) I 0c LIGHT CONTROL UTILIZATION AMPLIFIER DEVICE ('5 E 8 IO g \2 I0 5 Z s 0) OJ as E am I 0.I IO 2 4 6 BIG 20 50 I0 ILLUMINATION (FOOT CANDLES) INVENTOR.

Ronald A. Mancini AT TORNE Y PATENTEU ras H972 3.639.768

SHEET 2 BF 2 Fig.

lNV-ENTOR. Ronald A. Mancini yaw l ATTORNEY FEEDBACK CONTROLLED PHOTOSENSITIVE OBJECT DETECTING SYSTEM BACKGROUND OF THE INVENTION This invention relates to an object detecting system employing a photosensitive resistance to detect the presence of an object which attenuates'or completely blocks a beam of light that normally impinges upon the photosensitive resistance. More particularly, this invention relates to such an object detecting system, the operation of which is not adversely affected by changes in ambient conditions such as relative humidity, temperature and ambient light. This invention is particularly applicable to the detection of objects made of materials such as clear lime glass, clear plastic and the like, which objects transmit a high percentage of the light incident thereon. Object detection systems commonly employ a photocell and a resistor serially connected to form a simple voltage divider circuit. As the resistance of the photocell changes in response to a change in illumination, the voltage at the junction of the photocell and its associated resistor correspondingly changes. This change in junction voltage may be amplified and utilized to operate a utilization device such as a counter, an industrial control device or the like. In addition to the generation of voltage pulses indicative of the presence of an object between the photocell and the light source, low-frequency signals are generated at the junction due to ambient conditions changing the photocell resistance. An AC amplifier having zero DC gain is therefore utilized to amplify the change in junction voltage, thereby maintaining the system output nonresponsive to small photocell resistance changes caused by changing ambient conditions.

When the signal supplied to this amplifier by the photocell is at a sufficiently high level, the gain thereof can be kept relatively low, thereby causing the amplifier to be highly stable and insensitive to spurious signals. However, due to the existence of primarily two conditions, the signal supplied to the amplifier may be undesirably small. First, the object may be so transparent that the photocell illumination drops only a few percent when an object interrupts the light from the source. Secondly, the photocell may be operated at a point on its curve of cell resistance v. light intensity such that only a small change in voltage is produced by a given change in illumination. The first of these conditions is determined by the object and obviously cannot be controlled.

The intensity of the light source can be initially adjusted so that the photocell operates on a portion of its cell resistance v. light intensity curve which will provide an adequate change in voltage for the encountered change in illumination due to the presence of an object in the light path. To a limited extent the gain of the amplifier can be increased to compensate for that decrease in signal amplitude which is caused by small changes in ambient conditions. However, an increase in amplifier gain beyond a given level tends to make the amplifier unstable and susceptible to electrical noise. Large changes in ambient conditions would require excessively high amplifier gains which would be intolerable.

SUMMARY OF THE INVENTION It is therefore an object of this invention to provide an object detection system which properly functions over a wide range of ambient conditions without requiring operator adjustment.

A further object of this invention is to provide an object detection system which reliably senses the presence of objects which are made of highly transparent materials under wide ranges of ambient conditions.

Briefly, the object detection system in accordance with the present invention comprises a light source, photosensitive re sistance means located in receiving relationship with respect to the light source and an impedance device connected to the responsive only to relatively rapid changes in the voltage appearing at thisjunction. This invention is characterized in that feedback means responsive to slower changes in the voltage appearing at this junction controls the intensity of the light source and thereby compensates for changes in ambient conditions which would affect the operating characteristics of the photosensitive resistance.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of a prior art object detection system.

FIG. 2 is a schematic diagram of an object detection system in accordance with the present invention.

FIG. 3 is a typical photocell resistance curve.

FIG. 4 is a schematic diagram of the DC light control amplifier which is illustrated in block diagram form in FIG. 2.

DETAILED DESCRIPTION In the prior art system of FIG. 1 the series connection of a resistor 11 and a photocell 12 is connected between a DC voltage source 13 and reference potential. The photocell 12 is in light receiving relationship with respect to a light source 14 which is powered by a voltage source 15. The junction l7 between the photocell and the resistor 11 is connected to an AC amplifier 18, the output of which drives a utilization device.

A specific example will be given to illustrate the effect of ambient conditions on the voltage generated at the terminal 17. For the purpose of this example it is assumed that a resistor 11 is 200 k0. and that the photocell has a cell resistance curve 35 which is illustrated in FIG. 3. The voltage at the terminal 13 is 30 volts DC. If the photocell initially receives an illumination of 2.5 Ft. C., the resistance thereof is determined by the curve 35 to be kQ. The voltage at the terminal 17 will therefore be 10 volts. When a piece of lime glass interrupts the light beam, it will attenuate the light by approximately 8 percent. The light incident on the photocell is now 2.3 Ft. C., and the resistance thereof is increased to The voltage at the terminal 17 is correspondingly increased to 1 L2 volts. The increase in voltage at the terminal 17 due to the presence of a piece of lime glass in the space between the light source 14 and the photocell 12 is therefore 12 volts.

Continuing the above example, assume that ambient conditions change the resistance of the photocell to k!) in the absence of the piece of glass. An 8 percent decrease in the light illuminating the photocell will now cause a voltage increase of 0.55 volt at the terminal 17. The gain of the amplifier 18 can be made high enough to safely amplify this 0.55-volt signal. g

' However, when changes in ambient conditions become too large, the differential voltage developed at the terminal 17 may become so small that it cannot be amplified unless the amplifier gain is adjusted to be so high that it becomes unstable and vulnerable to spurious signals. For example, assume that the ambient light increases 100 percent from 2.5 to 25 Ft. C. Using the curve in FIG. 3 it can be seen that the photocell resistance will change from 21.5 k!) to 23 k0. when a piece of lime glass interrupts the light beam. The voltage differential at the terminal 17 now becomes 0.l6 volt. If the ambient temperature increases by 50 C. at the same time that the ambient light increases 100 percent, the photocell resistance will change from 18.7 k!) to l9.2 k0. in the presence of the glass object. The voltage differential at the terminal 17 would now be only 0.08 volt. The amplifier gain cannot be increased sufficiently to amplify this small change in voltage without jeopardizing the stability of the system.

The circuit shown in FIG. 2 overcomes the problems inherent in the prior art circuit of FIG. 1 by automatically adjusting the resistance of the photocell as it tends to change in response to slow changes in ambient conditions. Elements shown in FIG. 2 which correspond to elements in the circuit shown in FIG. 1 are designated by primed reference numerals.

The voltage at the terminal 17' is coupled to the input ofa DC light control amplifier which drives the light source 14'. The amplifier 20 must have zero AC gain and sufficient DC gain to maintain the resistance of the photocell 12 constant. In response to any slow change in ambient conditions such as temperature or light, the amplifier 20 will increase or decrease the illumination on the photocell 12 to compensate for the original change. This slow change in illumination incident upon the photocell 12' will not trigger the AC amplifier 18'. However, when a piece of glass breaks the light beam, the AC amplifier 18 will be triggered, but the DC light control amplifier 20 will not react to the fast change in the voltage at the terminal 17'.

It is thus seen that the object detecting system in accordance with this invention is self-compensating with respect to slow changes in ambient conditions and yet is sensitive enough to react to the presence of a piece of clear lime glass.

HO. 4 is a schematic diagram of an amplifier which may be used as the DC light control amplifier 20. Those elements in this figure which are similar to elements in FIG. 1 are designated by double primed reference numerals. Transistors 41, 42, 43 and 44 along with their associated biasing networks constitute a differential amplifier. The base of the transistor 41, which is the input terminal of the differential amplifier, is connected to the terminal 17". A transistor 45, a zener diode 46 and their associated biasing resistances and voltages constitute a constant current generator for the differential amplifier. The output of the differential amplifier, which appears at the terminal 48, is amplified by three transistor amplifiers which comprise the transistors 49, 50 and 51. A capacitor 53 having a high value of capacitance provides this amplifier with a very long time constant so that it responds only to very slow changes in voltage. Although the time constant of the circuit can be adjusted by a variable resistor 52, the primary purpose of resistor 52 is to determine the operating point of the photocell. Since the light source 14" is serially connected between a voltage source 55 and the collector of the transistor 51, the light output therefrom is determined by the conductivity of the transistor 51.

The system is initially adjusted so that the DC voltage existing at the terminal 17" is such that the photocell 12" is operating at the desired portion of its cell resistance curve. Slow changes in ambient conditions tend to cause the voltage at the terminal 17" to vary from this preselected value. The voltage at the terminal 17 is compared with that at the base of the transistor 44, and when these voltages become dissimilar, the voltage at the terminal 48 varies in such a manner that the conductivity of the transistor 51 changes. This changes the intensity of the light radiated by the light source 14" which in turn changes the resistance of the photocell 12" and restores the voltage at the terminal 17" to the initially selected value.

The amplifier illustrated in H6. 4 is merely illustrative of the DC light control amplifier which may be used in this invention. It will be obvious to those skilled in the art that other amplifiers may be used to maintain a constant voltage across the photocell. The voltage control circuit in accordance with this invention may be advantageously used in conjunction with any nonlinear photosensitive device to insure the operation of such device at the desired portion of its curve of cell resistance v. light intensity curve.

I claim:

1. A system for detecting in a given space the presence of objects that transmit a high percentage of light directed thereon, said system comprising:

a source of reference potential a first source of DC biasing potential having a terminal connected to said source,

a photocell disposed adjacent to said given space and having a terminal connected to said source,

light source means for directing a continuous light beam toward said photocell, said given space being located between said photocell and said light source means, a resistor connected between said photocell and said first source, the junction of said resistor and said photocell constituting a first terminal,

AC amplifier means having an input terminal directly connected to said first terminal, said AC amplifier means being responsive to fast changes in voltage at said first terminal due to an object passing through said given space,

utilization means connected to the output of said AC amplifier means,

a source of constant DC potential,

a differential amplifier having a first input terminal connected to said source of constant DC potential and a second input terminal connected directly to said first terminal, and further having means supplying a constant current thereto,

a first DC amplifier connected to the output of said differential amplifier,

capacitor means connected between the output of said first DC amplifier and said source of reference potential for providing along time constant impedance so that said DC amplifier responds only to slow changes in voltage at said first terminal,

variable resistor means connected in parallel with said capacitor means for varying the operating point of said photocell, and

direct current coupling means connecting the output of said first DC amplifier to said light source means, said first DC amplifier functioning in conjunction with said differential amplifier to maintain the voltage at said first terminal at a substantially constant level to thereby compensate for changes in ambient conditions which would affect the operating characteristics of said photocell, said direct current coupling means comprising a first transistor having base, emitter and collector electrodes, said first transistor base electrode being directly connected to the output of said first DC amplifier, and a second transistor having base, emitter and collector electrodes, said second transistor base electrode being directly connected to said first transistor emitter electrode and said second transistor collector electrode being directly connected to said light source means. 

1. A system for detecting in a given space the presence of objects that transmit a high percentage of light directed thereon, said system comprising: a source of reference potential a first source of DC biasing potential having a terminal connected to said source, a photocell disposed adjacent to said given space and having a terminal connected to said source, light source means for directing a continuous light beam toward said photocell, said given space being located between said photocell and said light source means, a resistor connected between said photocell and said first source, the junction of said resistor and said photocell constituting a first terminal, AC amplifier means having an input terminal directly connected to said first terminal, said AC amplifier means being responsive to fast changes in voltage at said first terminal due to an object passing through said given space, utilization means connected to the output of said AC amplifier means, a source of constant DC potential, a differential amplifier having a first input terminal connected to said source of constant DC potential and a second input terminal connected directly to said first terminal, and further having means supplying a constant current thereto, a first DC amplifier connected to the output of said differential amplifier, capacitor means connected between the output of said first DC amplifier and said source of reference potential for providing a long time constant impedance so that said DC amplifier responds only to slow changes in voltage at said first terminal, variable resistor means connected in parallel with said capacitor means for varying the operating point of said photocell, and direct current coupling means connecting the output of said first DC amplifier to said light source means, said first DC amplifier functioning in conjunction with said differential amplifier to maintain the voltage at said first terminal at a substantially constant level to thereby compensate for changes in ambient conditions which would affect the operating characteristics of said photocell, said direct current coupling means comprising a first transistor having base, emitter and collector electrodes, said first transistor base electrode being directly connected to the output of said first DC amplifier, and a second transistor having base, emitter and collector electrodes, said second transistor base electrode being directly connected to said first transistor emitter electrode and said second transistor collector electrode being directly connected to said light source means. 